The second year of investigation on this project has been largely concerned with efforts to integrate information from differential interference-contrast and fluorescence microscopy with that from ultrastructural examination of the ADH hydroosmotic response in the toad urinary bladder. Initial findings were that an important aspect of ADH action is to increase the compliance of the mucosal plasma membrane of granular cells so that swelling during water flow is primarily in the apical pole of these cells. Apparently related to this is a resulting asymmetry in cytoplasmic solute concentration during water flow so that vital organelles are protected from osmotic damage during a doubling of cell volume. Application of acridine orange (a fluorescent indicator of transmembrane pH gradients) with acidification of the transported water has suggested a transient compartmentalization of fluid in intracellular vacuoles; inconsistency of the vacuolation response itself requires further investigation. Inhibition of ADH-action with atropine and carbamyl choline seems consistent with the hormone's action as a relaxing agent for subapical contractile filaments.